Abstract
The frost survival mechanism of vegetative buds of angiosperms was suggested to be extracellular freezing causing dehydration, elevated osmotic potential to prevent freezing. However, extreme dehydration would be needed to avoid freezing at the temperatures down to −45°C encountered by many trees. Buds of Alnus alnobetula, in common with other frost hardy angiosperms, excrete a lipophilic substance, whose functional role remains unclear. Freezing of buds was studied by infrared thermography, psychrometry, and cryomicroscopy. Buds of A. alnobetula did not survive by extracellular ice tolerance but by deep supercooling, down to −45°C. An internal ice barrier prevented ice penetration from the frozen stem into the bud. Cryomicroscopy revealed a new freezing mechanism. Until now, supercooled buds lost water towards ice masses that form in the subtending stem and/or bud scales. In A. alnobetula, ice forms harmlessly inside the bud between the supercooled leaves. This would immediately trigger intracellular freezing and kill the supercooled bud in other species. In A. alnobetula, lipophilic substances (triterpenoids and flavonoid aglycones) impregnate the surface of bud leaves. These prevent extrinsic ice nucleation so allowing supercooling. This suggests a means to protect forestry and agricultural crops from extrinsic ice nucleation allowing transient supercooling during night frosts.
Highlights
Overwintering vegetative buds of temperate trees have been reported to survive freezing temperatures by different frost survival strategies (Sakai & Larcher, 1987)
Differential thermal analysis (DTA) plots obtained on buds of A. alnobetula during controlled freezing as a rule showed only a single freezing exotherm (HTE; Figure 4)
DTA and Infrared differential thermal analysis (IDTA) results suggest that the buds of A. alnobetula remain deeply supercooled down to below the frost killing temperature
Summary
Overwintering vegetative buds of temperate trees have been reported to survive freezing temperatures by different frost survival strategies (Sakai & Larcher, 1987). During deep supercooling of buds, water migration from the supercooled bud tissues to spaces outside, that is, the subtending stem or the bud scales, has been observed This process of freeze dehydration and external formation of ice masses has been termed extra‐organ freezing (Ishikawa & Sakai, 1982; Ishikawa & Sakai, 1985) and has been functionally associated with the maintenance of a supercooled state (Ide et al, 1998; Sakai & Larcher, 1987). Only recently it was shown that low midwinter MPa values of P. abies bud tissues have no effect on their supercooling capacity (Kuprian, Munkler, Resnyak, & Neuner, 2018) From these findings, the functional involvement of freeze dehydration or at least a dose‐effect relationship between freeze dehydration and freezing resistance of buds is not so clear. (3) we determined whether and—if so—buds get freeze dehydrated by movement of water towards the stem or bud scales and to what extent. (4) the findings should be related to the potential functional role of bud excretions in bud freezing resistance
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